1. Field of the Invention
The present invention relates to an RFID tag distance measuring system and a reader by which a distance between a reader and a Radio Frequency IDentification (RFID) tag can be measured and, in particular, an RFID tag distance measuring system and a reader by which a distance between a reader and an RFID tag can be securely measured.
2. Description of the Related Art
A conventional reader by which a distance from an RFID tag can be measured is disclosed in EP1239634 (Patent Document 1), for example. Patent Document 1 includes using the fact that the phase of a reflected wave from an RFID tag in response to a carrier signal from a reader depends on the distance between the reader and the RFID tag, demultiplexing the reflected wave into an I-signal and a Q-signal having phases identical to and orthogonal to that of the carrier signal, calculating the magnitude and phase of the reflected wave from the RFID tag, and calculating the distance from the RFID tag based on the phase.
Patent Document 1: EP1239623 (seventh column and FIG. 3, for example)
The construction has been adopted in a conventional reader by which a distance from an RFID tag can be measured. When the reader only receives a reflected wave directly from an RFID tag, the distance between the reader and the RFID tag can be measured by using the method disclosed in Patent Document 1. However, in reality, reflected waves from the RFID tag may include not only direct waves but also waves through paths reflected by a wall, a ceiling and so on. In other words, the reflected waves from an RFID tag may follow many paths (the environment of which is called multipath environment). In such a multipath environment, most reflected waves may not be received from an RFID tag even by transmitting signals at multiple different frequencies to the RFID tag in order to calculate the distance to the RFID tag. However, the point has not been considered in the past.